Usually, cotton fibers are produced by cultivating a cotton plant of the genus Gossypium and collecting the cotton fibers from the capsules (cotton bolls) formed on the cotton plant. There are many varieties of cotton plant, from which cotton fibers with different fiber characteristics can be obtained and used for various applications depending on their fiber characteristics. Cotton fibers are characterized by various properties among which fiber length, fiber fineness and fiber strength are particularly important. Many previous efforts have been made to improve the characteristics of cotton fibers. Attempted improvements have been mainly focused on fiber length and fiber fineness. In particular, there has been a great demand for longer and finer cotton fibers. The variety of cotton plant known as Sea Island is famous for desired fiber characteristics; however, this variety of cotton plant exhibits a poor yield of cotton fibers, therefore the price of Sea Island cotton fibers is very high. If highly yielding cotton plants with fiber characteristics equal to or better than those of Sea Island cotton can be produced, it will be a great contribution to industry.
The methods for improving the characteristics or yield of cotton fibers can be roughly classified into the following three categories:
1. Variety improvement by cross breeding
This method has been utilized most widely so far. At present, almost all the cultivated varieties of cotton plant were bred by this method. However, much time is needed for this method, and because of a limit to the degree of variability, one cannot expect remarkable improvements in fiber characteristics or in yield of cotton fibers.
2. Treatment with plant hormone
Plant hormones such as auxin, gibberellin, cytokinin and ethylene have been widely applied to field crops or horticultural products. Many reports have hitherto been made with respect to the influence of plant hormones on the fiber production of cotton plants, particularly on the fiber elongation mechanism. It is believed that fiber elongation is induced by gibberellin or auxin but inhibited by abscisic acid (Bhardwaj and Sharma, 1971; Singh and Sing, 1975; Baert et al., 1975; Dhindsa et al., 1976; Kosmidou, 1976; Babaev and Agakishiev, 1977; Bazanova, 1977; DeLanghe et al., 1978). Also Beasley and Ting Amer. J. Bot., 60(2): 130-139 (1973)! have reported that gibberellin has a promoting effect on the fiber elongation in ovule cultures (in vitro) whereas kinetin and abscisic acid have an inhibitory effect on the fiber elongation.
In a field test (in vivo), when non-fertilized flowers of cotton plants were treated with gibberellin just after flowering, there was found a promoting effect on the fiber elongation to a certain degree; in the case of fertilized flowers, however, no significant promotion was caused by gibberellin treatment (The Cotton Foundation Reference Book, Series Number 1, Cotton Physiology, 369, The Cotton Foundation, 1986).
As to the influence of plant hormones on the yield of cotton fibers was analyzed by MaCarty and Hedin who reported as follows: a field test on commercial plant growth regulators were conducted for a period of from 1986 to 1992. They found only in the field test of 1992 that an increase in fiber yield was observed with a Foliar Trigger (manufactured by Westbridge Chemical Co.) containing cytokinin or with FPG-5 (manufactured by Baldridge Bio-Research, Inc.) containing cytokinin, indoleacetic acid and gibberellin; however, no significant increase in fiber yield was observed in the other years J. Agric. Food Chem., 42: 1355-1357 (1994)!.
As described above, for the purpose of improving the characteristics and yield of cotton fibers, a number of studies and reports have been made on conventional plant hormones such as auxin, gibberellin, cytokinin and abscisic acid; however, no effect has been fully confirmed yet, and it cannot be said that these plant hormones are effective for practical use.
In recent years, much attention has been paid to brassinosteroids as a novel group of plant hormones, and the action of these hormones on various plants has been studied. For the first time, Mitchell, Mandave, et al., discovered brassinolide, which is one of the brassinosteroids, from Brassica napus pollen Nature, 225, 1065 (1970)!, and they confirmed that it has a remarkable effect on the cell elongation in the young buds of kidney bean. As described above, brassinolide is one of the steroid compounds with complicated structure, and many compounds with structural similarities thereto have since been discovered from various plants.
The effects of brassinosteroid when applied to cotton plants, was reported by Luo et al. Plant Physiology Communications, 5: 31-34 (1988)! that the treatment of boll stalks with 0.01 or 1 ppm brassinolide did not reduce the shedding of young bolls in a field test (in vivo). However, no report has hitherto been made that the characteristics or yield of cotton fibers can be improved by use of any brassinosteroid.
For callus culture (in vitro), Wang et al. Plant Physiology Communications, 28(1): 15-18 (1992)! reported that the addition of 0.01 ppm brassinolide to MS medium induced the callus formation and embryogenesis in cotton plants. However, no report has hitherto been made that the characteristics or yield of cotton fibers can be improved by addition of a brassinosteroid to a medium used for the ovule culture in the production of cotton fibers.
3. Variety improvement by gene recombination technique
In recent years, gene recombinant technique has made startling progress, and several reports have been made on the successful variety improvement in certain kinds of plants (e.g., tomato, soybean) by introduction and expression of a particular gene in these plants to confer a desired genetic trait thereon. If a gene associated with fiber formation and elongation can be introduced into cotton plants and expressed in large quantities, it would become possible to make a remarkable improvement in the characteristics or yield of cotton fibers. At present, however, only the following studies have been made on cotton plants: one is to improve insect resistance by introduction of a gene coding for BT toxin (Bacillus thuringiensis produced insecticidal protein toxin), and the other is to improve herbicide (Glyphosate) resistance by introduction of a gene coding for 5-enol-pyruvilshikimic acid 3-phosphate synthetase. These attempts result in an improved yield of cotton fibers per unit area but do not contribute to the improvement in the yield of cotton fibers per plant. The mechanism of fiber formation and elongation in cotton plants has not yet been fully elucidated and also very little is now known as to what genes are associated therewith.